Various methods for polymerizing a monomer having a vinyl group have been known, and an emulsion polymerization method and a suspension polymerization method are exemplified as an important method. The methods have advantages that the heat generated by a polymerization reaction can be easily removed and a reaction temperature can be readily controlled, since an aqueous solvent is used in the methods. In addition, the methods also have advantages that a polymer as a product can be obtained as a small particle and easily separated from a solvent phase, washed and dried.
In an emulsion polymerization method, a monomer may be polymerized using a surfactant and a polymerization initiator in an aqueous solvent. A monomer having a vinyl group is generally insoluble or hardly dissolved in water but is dispersed in an aqueous solvent by being incorporated in a micelle composed of a surfactant. In such a micelle, a monomer is polymerized by a radical generated from a polymerization initiator. The general size of the micelle is about several nanometers, and the polymer obtained by an emulsion polymerization method has excellent properties that the polymer is very fine as the size thereof is several dozen nanometers to several hundred nanometers but the polymerization degree thereof is large.
In addition, by an emulsion polymerization method, it is possible to adjust the composition, molecular weight and higher-order structure of a polymer depending on the use application thereof. Furthermore, a polymer dispersion obtained by an emulsion polymerization method is directly used as a paint, an adhesive, a medical or industrial determination agent or the like in some cases. Alternatively, a particulate polymer separated from an aqueous solvent after an emulsion polymerization is used, for example, for improving an impact resistance, toughness and melt processability, adjusting an optical property such as light diffusion, matte, interference color, coloration and wavelength cutting, and improving a blocking property of a film material by adding the particulate polymer to other material such as printing ink material, spacer and other polymer material.
On the one hand, a polymer obtained by an emulsion polymerization method may have a property intrinsic to the polymer lost due to the incorporation of a surfactant. A surfactant is also incorporated in a drainage after separating a polymer and causes environmental damage in some cases; therefore, it is required to reduce the amount of a surfactant. A prescribed amount of a surfactant is however needed for dispersing a micelle or a droplet of a monomer when a monomer which is insoluble or hardly dissolved in water is used in an emulsion polymerization.
As a biosurfactant, which is a surfactant derived from organisms, surfactin has been known. Surfactin exhibits an excellent surfactant activity, since surfactin has a cyclic peptide structure and the size of the hydrophilic cyclic structure is much larger than a conventional surfactant. Accordingly, a sodium salt thereof is used as a surfactant in cosmetics or the like (Patent Document 1).
In Patent Documents 2 to 9, a surfactin salt is exemplified as a surfactant used in an emulsion polymerization method. In addition, a method for polymerizing a vinyl-based monomer using a surfactin salt in a low concentration has been developed as Patent Document 10.